KVM: x86: Use "new" memslot instead of userspace memory region

Get the number of pages directly from the new memslot instead of
computing the same from the userspace memory region when allocating
memslot metadata.  This will allow a future patch to drop @mem.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <ef44892eb615f5c28e682bbe06af96aff9ce2a9f.1638817639.git.maciej.szmigiero@oracle.com>